1. Field of the Invention
This invention relates to strain sensors. The invention relates more particularly to a strain sensor design that has increased accuracy and that has improved manufacturability.
2. Description of Related Art
The United States government has identified air bag inflation control as an important area for development. It has mandated that thirty-five percent of automobiles sold in the United States starting in model year 2004 incorporate a system capable of detecting the weight of the occupant and deploying the air bag with varying force depending on that weight.
One approach to measuring occupant weight relies on a load bearing member in the seat structure having a strain-dependent electric resistance or strain sensor. The change in electrical resistance of the load bearing member serves as an indication of occupant weight.
Strain-dependent resistance may be provided by thick film resistive systems in conjunction with strain sensitive paints. Such paints, sometimes also referred to as “inks” or “resistive inks”, may be formulated so that dopants are present in boundaries where ruthenium crystals sinter during processing. At these points, as the overall structure is compressed, better contact is made between the crystals reducing the resistance of the material. Conversely, as the material is flexed, less contact is made and the resistance of the overall material is increased.
It is common in the art to attempt to maximize the amount of the strain sensitivity of the resistive ink materials. This is done to provide as high of a signal level as possible for the output from the device. Increased sensitivity is especially important for designs with ceramic substrates which are brittle and can be damaged with excessive bending. Thus, the amount of flexing or compression may be limited, resulting in a corresponding reduction in the output range available in the sensor
Generally, the paint materials are applied on an electrically insulative substrate in order to be used. If the paint were applied to a conductive material, the substrate would effectively provide a short circuit across the resistive material, rendering the sensor useless. Ceramic substrates have been used effectively to provide stress measurement, for example in use as pointing devices for laptop computers as described in U.S. Pat. No. 5,966,117 or as shear beam load cells as described in U.S. Pat. No. 6,225,526. However, ceramic materials are not adequate in some applications such as seat weight sensors where excessive stress or bending can occur. In these situations the ceramic substrate will break and the output signal will no longer be representative of the strain being measured. Thus, it would be advantageous to utilize a stronger material such as stainless steel in a load-bearing strain sensor.
It is known to provide a layer of porcelain as an insulative layer for screened resistive materials on steel. The steel provides a much improved mechanical substrate for the resistive paint materials. For example, U.S. Pat. No. 5,959,214 teaches providing a coating layer over the dielectric material before adding the strain sensitive materials.
Other examples of strain sensors are shown in United States Patents and Patent Publication Nos. 6,407,350, 6,407,347, 6,244,116, 6,231,076, 6,161,891 and 2002/0166385.
One problem that is common to all types of substrate materials used in strain sensors is warping and lack of flatness. Since most substrates are pressed or stamped, imperfections in the substrate material or stamping die can introduce unwanted variations in the thickness and planarity of the substrate during manufacturing. Unfortunately, a substrate that has as little as 0.001 inch of warp across a 1-inch diameter substrate can cause a strain resistor to be inaccurate and have a non-linear output.
A possible solution to planarize the substrates is to machine the substrates after stamping. Unfortunately, machining each substrate is costly, time consuming and introduces excessive cycle time for a high volume manufacturing operation.
A current unmet need exists for a cost effective strain sensor that can provide a linear output while at the same time accommodating substrate warping.